The goal of this research is to: 1) demonstrate and measure the surface tension gradient along the airway, particularly along parts of the bronchial tree where it has not been possible to measure surface tension in the past; 2) determine how the gradient changes with breathing; 3) demonstrate that the existence of the gradient is facilitated by the geometry of the distal airway; and 4) assess the efficacy of re- establishing normal surface tension gradients in surfactant-depleted lungs by administering exogenous surfactant substitutes. The investigators have demonstrated that rapid freezing and low- temperature scanning electron microscopy can preserve the alveolar aqueous lining layer in situ and that the layer is continuous and thin, averaging 0.2 microns. As an extension of this research, the investigators have recently developed the following new method for measuring surface tension along the airway. Anaesthetized rats are ventilated with a nebulized mist of fluorocarbon liquid via an endotracheal tube connected to a pressure controlled respirator. After 20 minutes, lungs are frozen in situ with a cryoprobe, maintained at liquid nitrogen temperature, and then removed and transferred to the low-temperature scanning electron microscope where the surfaces of freshly exposed airways are imaged with water and fluorocarbon present. The investigators have shown in experiments on lungs that submicrometer fluorocarbon droplets are deposited along the airway out to subpleural alveoli. The investigators used a specially-equipped Wilhelmy surface balance to show that freezing does not change droplet shape and that droplet contact angle assessed with the low-temperature scanning electron microscope is a measure of surface tension in the airway in vivo. These measurements of surface tension are intended to fill the gap between those obtained in the outermost alveoli (of as low as zero dynes/cm in isolated lungs) and in central airways (of 32 dynes/cm in vivo).The investigators will now be able to test whether there is a gradual change in surface tension along the airway, which could, for example, aid in the clearance of surface- associated particles from the lung.The investigators will determine the volume and respiratory pattern-dependence of the gradient by measuring it at large versus small lung volumes and following periods of high, normal and low ventilation with varying rates and depths. The investigator will study the viscous behavior of lung surfactant preparations in a Wilhelmy surface balance with a long, shallow, agarose-lined channel in the barrier to demonstrate that a surface tension gradient can be maintained by models of known geometry of natural anatomical monolayer escape paths. The investigators will quantify the gradient in clinical surfactant deficiency by washing lungs and then administering exogenous surfactant substitutes to gauge their efficacy in re- establishing normal surface tension gradients. This work is relevant to clinical deficiencies of surfactant such as the Respiratory Distress Syndrome and to understanding current problems with surfactant replacement therapy.